Pulmonary endothelial, type II and probably mesenchymal interstitial cells in the neonatal animal interact in the lipolysis of chylomicrons and in the reesterification of lipids to produce surfactant. Surfactant is a requisite for successful respiration. At the time of birth deficient production of surfactant results in Respiratory Distress Syndrome and is the principal problem associated with premature infant mortality. Ultrastructural cytochemical and morphometric techniques will be used to identify the sites of lipolysis and lipid reesterification in the pulmonary endothelial, mesenchymal interstitial cells of rats of various postnatal ages. The cytochemical and morphometric comparison of lung from fed, fasted and refed rats will provide information regarding the response of mesenchymal interstitial cells to the needs of type II cells during fasting. The relationship of endothelial lipase to other enzymes will be examined. In developing muscular pulmonary arteries and veins the relations of the potentially lipolytic endothelium to lipid deposition in smooth muscle cells or other cells of the vessel wall will be examined. The role of microtubules and hormones on the lipolytic and reesterification activities of the postnatal pulmonary cells will be examined. The information derived from this study will produce better understanding of the interrelation of the cells involved in providing materials for surfactant synthesis and this understanding will lead to improved diagnosis and treatment techniques.